Hydraulic fracturing is used to create subterranean fractures that extend from the borehole into rock formation in order to increase the rate at which fluids can be produced by the formation. Generally, a high viscosity fracturing fluid is pumped into the well at sufficient pressure to fracture the subterranean formation. In order to maintain the increased exposure to the formation, a solid proppant is added to the fracturing fluid which is carried into the fracture by the high pressure applied to the fluid.
Greater than 65% of conventional fracturing fluids are made of guar gum (galactomannans) or guar gum derivatives such as hydroxypropyl guar (HPG), carboxymethyl guar (CMG) and carboxymethylhydroxypropyl guar (CMHPG). These polymers can be crosslinked in order to increase their viscosities and increase their capabilities of proppant transport.
Once the high viscosity fracturing fluid has carried the proppant into the formation, breakers are used to reduce the fluid's viscosity which allows the proppant to settle into the fracture and thereby increase the exposure of the formation to the well. Breakers work by reducing the molecular weight of the polymers, thus ‘breaking’ the polymer. The fracture then becomes a high permeability conduit for fluids and gas to be produced back to the well.
Chemical oxidizers and enzymes are most commonly used as breakers. The oxidizer produces a radical which then degrades the polymer. This reaction is limited by the fact that oxidizers are stoichiometric and they will attack not only the polymer but any molecule that is prone to oxidation. Enzymes, on the other hand, are catalytic and substrate specific and will catalyze the hydrolysis of specific bonds on the polymer. An enzyme will degrade many polymer bonds in the course of its lifetime. Unfortunately, enzymes operate under a narrow temperature range and their functional states are often inactivated at high temperatures.
Conventional enzymes used to degrade galactomannans work well at ambient to moderate temperatures (75° F. to 150° F.). At elevated temperatures, (>150° F.) these enzymes quickly denature and lose activity. The beta-mannanase enzyme used in conventional enzyme formulations has a temperature maximum of approximately 150° F. Activity profiles have indicated that the enzyme retains little to no activity past this point. Because many downhole fracturing operations are conducted at temperatures in excess of 150° F., it would be beneficial to have an enzyme that can degrade guar-based fracturing fluids under these elevated temperatures.